1. Field of the Invention
This invention relates generally to particulate conductive carbons. The invention also relates to supported catalysts for fuel cells and proton exchange membranes.
2. Background
A fuel cell (FC) is a device that converts energy of a chemical reaction into electrical energy (electrochemical device) without combustion. A fuel cell (see e.g., FIG. 1) generally comprises an anode 20, cathode 50, electrolyte 10, backing layers 30, 60, and flow fields/current collectors 40, 70. There are five types of fuel cells, as defined by their electrolytes:
TypeElectrolyteTemperatureCommentsPhosphoricLiquid phosphoric acid175–200° C.Stationary power,acid (PAFC)soaked in a matrixcommercially availableMoltenLiquid solution of lithium,600–1200° C.Molten carbonate salts,carbonatesodium and/or potassiumhigh efficiency(MCFC)carbonates, soaked in amatrixSolid oxideSolid zirconium oxide to600–1800° C.Ceramic, high power,(SOFC)which a small amount ofindustrial applicationsytrria is addedAlkalineAqueous solution of 90–100° C.Potassium hydroxide(AFC)potassium hydroxide soakedelectrolyte, NASA,in a matrixvery expensive**ProtonSolid organic polymer 60–100° C.Ionomer membrane,exchangepolyperfluorosulfonic acidhigh power density, canmembranevary output quickly,(PEM)portable/autoapplicationsDirect 60–100° C.PEM that usesMethanolmethanol for fuel(DMFC)**= Currently of most interest
The current description deals with proton exchange membrane (a.k.a. polymer electrolyte membrane) (PEM) fuel cells (a.k.a. solid polymer electrolyte (SPE) fuel cell, polymer electrolyte fuel cell, and solid polymer membrane (SPM) fuel cell). A polymer electrolyte membrane fuel cell (PEMFC) comprises a proton conductive polymer membrane electrolyte 10 sandwiched between electrocatalysts (a cathode 50 and an anode 20) (see, e.g., FIG. 1).
The oxidation and reduction reactions occurring within the fuel cell are:2H2→4H++4e− oxidation half reaction+O2→2H2O reduction half reactionThis electrochemical process is a non-combustion process which does not generate airborne pollutants. Therefore, fuel cells are a clean, low emission, highly efficient source of energy. Fuel cells can have 2–3 times greater efficiency than internal combustion engines and can use abundant and/or renewable fuels. Fuel cells produce electricity, water, and heat using fuel 90 and oxygen 80. Water (liquid and vapor) is the only emission when hydrogen is the fuel.
Since the voltage of a typical fuel cell is small, they are usually stacked in series.
The two half-reactions normally occur very slowly at the low operating temperature of the fuel cell, thus catalysts 56 are used on one or both the anode 20 and cathode 50 to increase the rates of each half reaction. Platinum (Pt) has been the most effective noble metal catalyst 56 to date because it is able to generate high enough rates of O2 reduction at the relatively low temperatures of the PEM fuel cells. Kinetic performance of PEM fuel cells is limited primarily by the slow rate of the O2 reduction half reaction (cathode reaction) which is more than 100 times slower than the H2 oxidation half reaction (anode reaction). The O2 reduction half reaction is also limited by mass transfer issues.
As fuel 90, such as hydrogen, flows into a fuel cell on the anode side, a catalyst 56 facilitates the separation of the hydrogen gas fuel into electrons and protons (hydrogen ions). The hydrogen ions pass through the membrane 10 (center of fuel cell) and, again with the help of the catalyst 56, combine with an oxidant 80, such as oxygen, and electrons on the cathode side, producing water. The electrons, which cannot pass through the membrane 10, flow from the anode 20 to the cathode 50 through an external circuit containing a motor or other electrical load, which consumes the power generated by the cell.
A catalyst 56 is used to induce the desired electrochemical reactions at the electrodes 20, 50. The catalyst 56 is often incorporated at the electrode/electrolyte interface by coating a slurry of the electrocatalyst particles 56 to the electrolyte 10 surface. When hydrogen or methanol fuel 90 feed through the anode catalyst/electrolyte interface, electrochemical reaction occurs, generating protons and electrons. The electrically conductive anode 20 is connected to an external circuit, which carries electrons by producing electric current. The polymer electrolyte 10 is typically a proton conductor, and protons generated at the anode catalyst migrate through the electrolyte 10 to the cathode 50. At the cathode catalyst interface, the protons combine with electrons and oxygen to give water.
The catalyst 56 is typically a particulate metal such as platinum and is dispersed on a high surface area electronically conductive support 52.
The electronically conductive support material 52 in the PEMFC typically consists of carbon particles. Carbon has an electrical conductivity (10−1–10−2 S/cm) which helps facilitate the passage of electrons from the catalyst 56 to the external circuit. Proton conductive materials 54 such as Nafion® are often added to facilitate transfer of the protons from the catalyst 56 to the membrane interface.
To promote the formation and transfer of the protons and the electrons and to prevent drying out of the membrane 10, the fuel cells are operated under humidified conditions. To generate these conditions, hydrogen fuel 90 and oxygen 80 gases are humidified prior to entry into the fuel cell. In a supported electrocatalyst (52+56), carbon is relatively hydrophobic, and as such, the boundary contact between the reactive gases, water and the surface of the solid electrodes made of carbon contributes to high electrical contact resistance and ohmic power loss in the fuel cell resulting in lower efficiency of the fuel cell.
In the present invention, the hetero atom-containing conductive polymer-grafted carbon material shows hydrophilic character and thereby enhances the humidification process. Also, the higher electronic conductivity of these polymers facilitates the electron transfer process.
An ordinary electrolyte is a substance that dissociates into positively charged and negatively charged ions in the presence of water, thereby making the water solution electrically conducting. The electrolyte in a PEM fuel cell is a polymer membrane 10. Typically, the membrane material (e.g., Nafion®) varies in thickness from 50–175 μm. Polymer electrolyte membranes 10 are somewhat unusual electrolytes in that, in the presence of water, which the membrane 10 readily absorbs, the negative ions are readily held within their structure. Only the protons contained within the membrane 10 are mobile and free to carry positive charge through the membrane 10. Without this movement within the cell, the circuit remains open and no current would flow.
Polymer electrolyte membranes 10 can be relatively strong, stable substances. These membranes 10 can also be effective gas separators. Although ionic conductors, PEM do not conduct electrons. The organic nature of the structure makes it an electronic insulator. Since the electrons cannot move through the membrane 10, the electrons produced at one side of the cell must travel through an external circuit to the other side of the cell to complete the circuit. It is during this external route that the electrons provide electrical power.
A polymer electrolyte membrane 10 can be a solid, organic polymer, usually poly(perfluorosulfonic) acid. A typical membrane material, Nafion® , consists of three regions:    (1) the Teflon®-like, fluorocarbon backbone, hundreds of repeating —CF2—CF—CF2— units in length,    (2) the side chains, —O—CF2—CF—O—CF2—CF2—, which connect the molecular backbone to the third region,    (3) the ion clusters consisting of sulfonic acid ions, SO3−, H+. The negative ions, SO3−, are permanently attached to the side chain and cannot move. However, when the membrane 10 becomes hydrated by absorbing water, the hydrogen ions become mobile. Ion movement occurs by protons, bonded to water molecules, migrating from SO3− site to SO3− site within the membrane. Because of this mechanism, the solid hydrated electrolyte is a good conductor of hydrogen ions.
The catalyst support 52 serves to conduct electrons and protons and to anchor the catalyst 56 (e.g., noble metal). Many efforts have been aimed at lowering the costs of fuel cells by lowering noble metal (e.g., platinum) catalyst 56 levels due to noble metal's cost. One way to lower this cost is to construct the catalyst support layer 52 with the highest possible surface area.
The electrodes 20, 50 of a fuel cell typically consist of carbon 52 onto which very small metal particles 56 are dispersed. The electrode is somewhat porous so that gases can diffuse through each electrode to reach the catalyst 56. Both metal 56 and carbon 52 conduct electrons well, so electrons are able to move freely through the electrode. The small size of the metal particles 56, about 2 nm in diameter for noble metal, results in a large total surface area of metal 56 that is accessible to gas molecules. The total surface area is very large even when the total mass of metal 56 is small. This high dispersion of the catalyst 56 is one factor to generating adequate electron flow (current) in a fuel cell.
Conducting polymers are a class of conjugated double bond polymers whose electrical conductivities are comparable to the conductivities of semiconductors to metals, in the range of 0.1 to 100 S/cm. Typical examples of conducting polymers include polyaniline, polypyrrole, polythiophene, polyfuran, and polyphenylene. Both polyaniline and polypyrrole catalyst support 52 materials have shown improved fuel cell efficiency (e.g., U.S. Pat. No. 5,334,292 and WO 01/15253). However, the long-term stability of these materials has not been demonstrated in electrode environments in cyclic operations.
Conducting polymers alone used as catalyst support 52 material have higher costs, lower surface area, and lower stability compared to those supports 52 based on carbon.
An example of a current commercial carbon-supported catalyst for fuel cells is the HiSPEC™ series of products (Johnson Matthey, Reading, U.K.) which utilize Vulcan® XC72 (Cabot Corporation) carbon black loaded with various levels of platinum (or other metal). These commercial carbon-supported catalysts are very expensive.
Factors such as surface area and electronic conductivity have historically been viewed as important for the carbon support material. However, relatively little research has been undertaken to understand the role of or to optimize the carbon support.
In the present invention, a conducting polymer is grafted onto the surface of a carbonaceous material thereby increasing the electrical conductivity of the carbonaceous material, and the stability of the hybrid material is expected to be enhanced. The polymer grafting process also reduces the porosity of the carbon support, resulting in increased metal availability for electrode reaction.
The majority of the cost associated with electrodes is attributed to the high cost of the metal, which makes up the catalyst 56. Only those catalytic sites exposed on the surface of the catalytic particles contribute to the catalytic activity of the electrode and, thus, electrodes with the highest fraction of the metals accessible to the reaction should be the most effective. Carbon supports 52 with high porosity result in “trapped” metal sites that are not accessible for electrode reaction. The extent of dispersion of the metal catalyst 56 on the support material 52 and the stability of such high dispersion in use, i.e., resistance of the catalyst against sintering and/or agglomeration, is directly related to the surface area and the availability of surface sites on which the dispersed metal 56 can be anchored.
In the present invention, the conducting polymer-grafted carbon material aids the uniform dispersion and stabilization of metal particles by anchoring to hetero atoms, namely, N, O, S, etc., present in the conducting polymer. Also, the hetero atom-containing anchoring groups resist the agglomeration and sintering of metal (e.g., platinum (Pt)) crystallite particles.
It is desirable to provide a catalyst support 52 that has a higher surface area and also a higher surface density of anchoring surface sites than catalytic supports consisting exclusively of carbon. This would increase and stabilize the dispersion of the metal catalyst 56 and, thus, limit the amount of catalyst 56 needed. The present invention provides a PEMFC electrode which can be made more cost-effective than electrodes having exclusively carbon support or exclusively conducting polymer support.
For the above reasons, improvement of the supported catalyst is desired and has been achieved with the present invention.